Lignes de grains : Types, étapes, causes, effets (mise à jour 2023)
What are squall lines? How do they form? What types of squall lines exist? What are the dangers associated with squall lines? In this blog post, we will answer all of these questions and more.
Squall lines can be dangerous weather phenomena, particularly if you are caught in one without knowing what to do. We hope that this guide will help you stay safe during thunderstorms and squall lines!
As a pilot, you need to be aware of various types of aerial phenomena, and squall lines are one of them. So, let's take a detailed look at squall lines!
Table des matières
- What Are Squall Lines?
- How Do Squall Lines Form?
- Types of Thunderstorms
- Characteristics of Squall Lines
- Types of Squall Lines
- How Do Pilots Avoid a Squall Line?
- What Should You Do If You Encounter a Squall Line?
What Are Squall Lines?
Squall lines are areas of active thunderstorms that are usually several hundred miles long and 60 miles wide. These squall lines typically form along cold front boundaries in the spring and fall. However, they can also form along warm front boundaries and outflow boundaries from thunderstorms. They are called squall lines because they usually produce a line of severe thunderstorms.
Squall lines are found near a cyclone, which is an area of low pressure in the atmosphere. The cyclone helps to lift air upwards, which then cools and condenses into thunderstorms. The squall line forms along the leading edge of the cold front associated with the cyclone.
As squall lines move across the landscape, they can produce high winds, heavy rain, hail, and tornadoes. Therefore, it is important to be aware of squall lines and know how to stay safe if you find yourself in one.
How Do Squall Lines Form?
Squall lines typically form along cold front boundaries in the spring and fall. However, they can also form along warm front boundaries and outflow boundaries from thunderstorms. The main ingredient for squall line formation is a large temperature contrast over a short distance.
You need a large temperature contrast over a short distance for squall line formation. This can happen when a cold front moves through an area of warm, moist air. The cold front's leading edge acts as a lifting mechanism that helps lift the warm, moist air upwards.
As the air rises, it cools and condenses into thunderstorms. The squall line forms along the leading edge of the cold front associated with the cyclone.
Squall lines form during thunderstorms, which occur in three stages: Cumulus, mature, and dissipating.
The Cumulus (Developing) Stage
The cumulus stage is when the squall line is first forming, and there are cumulonimbus clouds present. Cumulonimbus clouds are thunderstorms that have reached the mature stage.
At this stage, you will likely see cumulus clouds on the leading edge of the squall line. These clouds are called cumulonimbus incus clouds. They are an indicator that the squall line is about to produce severe weather.
The Mature (Active) Stage
The squall line is in the mature stage when it is producing severe weather. This is the most dangerous time to be in a squall line. The squall line will usually be several hundred miles long and 50 to 60 miles wide. The squall line will have strong winds and heavy rains. There may also be large hail, thunderstorms, and tornadoes. The squall line will move quickly, usually at 30 to 40 mph.
The Dissipating Stage
The squall line will continue to produce severe weather until it dissipates. The squall line will dissipate when the warm air ahead of the squall line runs out of energy. The squall line will also dissipate if it moves into an area where there is no warm air. The squall line will usually dissipate within 12 hours after it forms.
Types of Thunderstorms
The squall lines play an important part in determining the type of thunderstorm it can be.
Single-cell thunderstorms are the most common type of thunderstorms. They usually last for 30 minutes to an hour. Single-cell thunderstorms are produced by unstable air rising on its own. The thunderstorm will dissipate when the updraft can no longer sustain itself. It is also called the "popcorn" convection due to the way it forms.
Multi-cell thunderstorms are more complex than single-cell thunderstorms. They are made up of several thunderstorms that merge together. Multi-cell thunderstorms can last for several hours. New updrafts form as the squall line moves.
The thunderstorms will continue to grow as long as there is enough instability and moisture. Multi-cell thunderstorms are accompanied by heavy rains, strong winds, and large hail.
Supercell thunderstorms are the most severe type of thunderstorms. They can last for several hours and produce tornadoes, large hail, and strong winds. Supercell thunderstorms are made up of a single storm cell that rotates around a central updraft. The rotation is caused by the Coriolis effect. Supercell thunderstorms usually form along squall lines.
Supercell thunderstorms last for a long time because the storm cell can tap into new sources of instability and moisture. The storm cell will also be reinforced by inflow from other thunderstorms.
Tropical thunderstorms are different from other types of thunderstorms. They are usually much larger and can last for days. Tropical thunderstorms form over warm ocean waters. They are fed by the warm, moist air that rises from the surface of the ocean. Tropical thunderstorms can become hurricanes when they reach land.
Characteristics of Squall Lines
Squall lines typically have a few characteristics that make them easy to identify:
- Squall lines usually form in the afternoon or evening.
- Squall lines are often associated with a drop in temperature, a decrease in air pressure, and an increase in wind speed.
- Squall lines often produce heavy rain, strong winds, hail, and tornadoes.
- Strong winds and heavy rains characterize the leading areas of squall lines. The squall line will also have a gust front. The gust front is the leading edge of the squall line where the winds are strongest. The gust front can extend several miles ahead of the squall line.
- The trailing areas of squall lines are characterized by light rain and weak winds. The squall line will also have an outflow boundary. The outflow boundary is the trailing edge of the squall line where the winds are weakest. The outflow boundary can extend several miles behind the squall line.
- The squall line will also have pressure perturbations. The pressure perturbations are areas of low pressure that form along the squall line. The low-pressure areas can extend several miles ahead of the squall line.
- The squall line will also have a wake low. The wake low is an area of low pressure that forms behind the squall line. The wake low can extend several miles behind the squall line.
- The squall line will also have a comma head. The comma head is an area of low pressure that forms at the leading edge of the squall line. The comma head can extend several miles ahead of the squall line.
- The squall line will also have a trailing stratiform region. The trailing stratiform region is an area of light rain and weak winds that forms behind the squall line. The trailing stratiform region can extend several miles behind the squall line.
Types of Squall Lines
Squall lines are classified based on the areas of lighter thunderstorms or rain and stratiform precipitation. Therefore, it is classified as trailing stratiform, leading stratiform, and parallel stratiform.
- A trailing stratiform squall line is a squall line in which the area of thunderstorms trails an area of stratiform precipitation.
- A leading stratiform squall line is a squall line in which an area of thunderstorms leads to an area of stratiform precipitation.
- A parallel squall line is a squall line in which an area of thunderstorms runs parallel to an area of stratiform precipitation.
The squall line is also classified according to its orientation with respect to the wind shear vector. If the squall line is oriented perpendicular to the wind shear vector, it is called a linear squall line. If the squall line is oriented parallel to the wind shear vector, it is called a curved squall line.
- Linear squall lines are composed of a series of thunderstorms that are lined up in a row. This type of squall line typically forms ahead of cold fronts and weakens as it moves into an area where the air is more stable.
- Curved squall lines form when winds at different levels are blowing at different speeds or directions. This type of squall line usually forms along jet stream boundaries.
Squall lines can also be classified according to their development. If the squall line forms on its own, it is called a discrete squall line. If the squall line forms along with other thunderstorms in an area of showers and thunderstorms, it is called a mesoscale convective system (MCS).
- Discrete squall lines typically form ahead of cold fronts and move quickly through an area.
- Mesoscale convective systems can last for several hours and Cover large areas. MCSs often produce heavy rain, strong winds, hail, and tornadoes.
How Do Pilots Avoid a Squall Line?
Due to the long line' nature of squall lines, they are often visible on weather radar images for considerable distances ahead of an aircraft's position. As squall lines generally move quite rapidly, it may be possible to outrun them if the sufficient warning is received.
Landing Prior to The Squall Line
Landing prior to the squall line's arrival is obviously the safest option to approach. In case of an incoming squall line, it is safest to land the plane mid-travel and resume once the squall has passed.
Diverting Around The Squall Line
In some cases, landing the plane in time might not be possible, or it is more efficient to fly around the squall line. In this case, pilots will attempt to divert around the squall line by flying at a higher altitude where the squall line is not as dense. In doing this, it is important to be aware of the squall line's movement and speed so that the plane does not end up flying into it.
What Should You Do If You Encounter a Squall Line?
Squall lines can form quickly and unexpectedly, so it is important to be aware of the signs of imminent squall line formation. These signs include a sudden drop in temperature, large cumulonimbus clouds, increasing wind speed, and hail.
If you see these signs, begin preparing for squall line flying and be ready to take action quickly if necessary, as squall lines are a serious threat to aviation and can cause significant damage to aircraft. It is important to be aware of squall lines' dangers and know how to avoid them.
If you are caught in a squall line while flying, the best course of action is to fly where the squall line is not as dense. If you are unable to do this, try turning around, as squall lines typically move from west to east.
However, in the unfortunate circumstance you are caught within a squall, do not turn the flight around, as this will only put you in the squall line for longer. If not, continue flying through the squall line and monitor your aircraft instruments closely.
Another recommendation is to adopt a holding pattern, which is an aerodynamic maneuver in which an aircraft flies a specific route in a circular or rectangular pattern until the squall line has passed.
If you intend to change direction, squall lines typically move from west to east, so changing your direction to the opposite direction of the squall line is usually the best option. Employ the artificial horizon, which is the most important instrument in squall line flying, to maintain level flight.
Additionally, inform the Air Traffic Controller if you are squall line flying so they are aware of your situation and can give you updated weather information. Ensure the autopilot is disengaged and be prepared for turbulence. If you have passengers on the flight, brief them on the situation and let them know what to expect. Inform that any loose items must be secure and turn on the seatbelt sign.
It is important to maintain a safe speed when flying through a squall line. squall lines can cause severe turbulence and downdrafts, which can damage your aircraft.
This will help you avoid the strongest turbulence and downdrafts. Anti-ice, especially the pilot heat, must be on, and windshield wipers must be ready if necessary.
After squall line flying, be sure to check your aircraft for any damage that may have occurred. If you encounter squall line flying, stay calm and follow the proper procedures to ensure a safe flight.
This blog post attempted to provide a comprehensive guide to squall lines. We hope you found it informative and helpful. It attempted to draw an idea of what squall lines are, what are their causes, types, and characteristics, and most importantly, how you can avoid squall line flying and basic steps to adopt if you do end up flying into a squall line.
While squall lines can be dangerous, they are also awe-inspiring phenomena that are worth observing from a safe distance. If you have the opportunity to witness a squall line, consider yourself lucky. Just be sure to stay safe and follow the proper procedures if you encounter squall line flying. Thanks for reading!